1. Field of the Invention
The invention relates to an exhaust emission control device of an internal-combustion engine, specifically to a technique of increasing efficiency in converting exhaust emissions using a three-way catalyst.
2. Description of the Related Art
As a catalyst for exhaust emission control of an internal-combustion engine for a vehicle, a three-way catalyst is generally used broadly.
Recently, in order to make full use of the capabilities of a three-way catalyst, a three-way catalyst having a so-called O2 storage component (hereinafter abbreviated to “OSC”) has been developed. In lean air-fuel ratio operation of an internal-combustion engine, the three-way catalyst having the OSC stores O2, thereby makes a catalytic atmosphere close to a stoichiometric air-fuel ratio temporarily and causes conversion of NOx in this catalytic atmosphere. Then, in rich air-fuel ratio operation of the internal-combustion engine, the three-way catalyst having the OSC releases the stored O2 and thereby accelerates conversion of HC and CO.
In this type of three-way catalyst having an OSC, A/F variation control in which lean air-fuel ratio operation is forcedly switched over to rich air-fuel ratio operation before the OSC becomes saturated is performed, so that O2 is stored and released repeatedly. By this, HC and CO as well as NOx can be converted efficiently.
Also, a three-way catalyst having a CO storage component (hereinafter abbreviated to “COSC”) has been developed. The three-way catalyst having the COSC accelerates conversion of NOx by storing CO in rich air-fuel ratio operation, and then releasing the stored CO in lean air-fuel ratio operation (Japanese Unexamined Patent Publication No. 2002-89250).
An experiment carried out by the inventors has confirmed that the three-way catalyst has a characteristic that when A/F variation control is performed, CO rather than HC is selectively converted in rich air-fuel ratio operation. Hence, even the three-way catalyst having an OSC has a problem that HC is not converted satisfactorily in rich air-fuel ratio operation (see FIG. 6).
Recently, for purposes such as improving fuel economy, a vehicle which can perform a so-called fuel cut, namely stopping of supply of fuel to the internal-combustion engine, for example, while decelerating is being developed and put into practical use. However, while a fuel cut is performed, O2 is discharged as it is, so that the O2 storage component easily becomes saturated. Thus, in the case of a three-way catalyst having an OSC, when rich air-fuel ratio operation is performed after resumption of fuel supply, a large quantity of O2 is released at once. Hence, even when the three-way catalyst has a COSC, the catalytic atmosphere becomes a lean atmosphere, which causes a problem called NOx spike, namely production of a large quantity of NOx.
Under the circumstances, it is thinkable to perform A/F variation control using a three-way catalyst having no OSC. In this case, in rich air-fuel ratio operation, HC rather than CO is converted relatively well, and in rich air-fuel ratio operation performed after resumption of fuel supply, the catalytic atmosphere's becoming a lean atmosphere is prevented.
However, use of a three-way catalyst having no OSC has a problem that CO produced in rich air-fuel ratio operation and NOx produced in lean air-fuel ratio operation cannot not sufficiently be converted (see FIG. 7).